Method and apparatus for receiving an amplitude modulated radiosignal

ABSTRACT

Receiving apparatus ( 1 ) for receiving an amplitude modulated signal which may be transmitted on a plurality of frequency channels within a predetermined frequency bandwidth comprises a tuner ( 3 ) for sequentially scanning through the channels under the control of a microprocessor ( 6 ) which determines the presence of a signal on a channel to which the tuner ( 3 ) is tuned. A first validation circuit ( 10 ) determines if the received signal is a valid type signal, and if so the microprocessor ( 6 ) counts the channels from the channel on which the signal is first detected to the channel on which the signal can no longer be detected is reached. The microprocessor ( 6 ) determines the middle one of the channels in which the signal is detected, and the microprocessor ( 6 ) locks the tuner ( 3 ) onto the frequency of the middle channel, and simultaneously operates a gate circuit ( 11 ) for outputting at least one complete signal to security apparatus of a motor vehicle for switching the state of the security apparatus from one of an armed and a disarmed state to the other of the armed and disarmed state. A noise suppressing circuit ( 9 ) is responsive to a noise analysing circuit ( 8 ) under the control of the microprocessor ( 6 ) for suppressing noise in the received signal.

[0001] The present invention relates to a method and apparatus forreceiving an amplitude modulated (AM) radio signal, and in particular,for receiving an AM radio signal which may be transmitted in any one ofa plurality of frequency channels within a predetermined frequencybandwidth.

[0002] Vehicle security apparatus is commonly operated remotely by aradio signal, which typically is transmitted by a hand-held transmitter.Such transmitters, are relatively small and are suitable for mounting ona key ring, and typically, are referred to as keyfob transmitters.Alternatively, the transmitter may be contained in the head of a key.Such security apparatus typically comprises a central control circuitwhich receives the radio signal from the transmitter through a radioreceiver associated with the central control circuit. Signals receivedby the radio receiver are analysed in the central control circuit toascertain if a signal is a valid signal. On reception of a valid signalthe central control circuit changes the state of the security apparatus.For example, the central control circuit may control a vehicleimmobiliser for immobilising the vehicle, the central locking system forlocking the doors of the vehicle, a vehicle alarm and the like. Thus, ifthe security apparatus is in an armed state, whereby the immobiliser isin a state for immobilising the vehicle, the central locking system isin a state with the doors locked, and the alarm circuit is armed fordetecting unauthorised entry or an attempted unauthorised entry to thevehicle, on reception of a valid signal the central control circuitdisarms the security apparatus thereby operating the central lockingsystem for unlocking the doors, operating the immobiliser to allow thevehicle to be mobilised, and for disarming the alarm circuit. On thesecurity apparatus being in the disarmed state, a valid signal receivedby the radio receiver causes the central control circuit to arm thesecurity apparatus.

[0003] Typically, the remote transmitter in each transmission transmitsa series of encoded signals, each of which comprises a preamble codewhich is common to a particular type of security apparatus, and an IDcode which is specific and unique to the specific security apparatuswhich is to receive the signal. Typically, one to five complete signalsare transmitted in each series.

[0004] It is not unusual to encounter areas in which certain frequencychannels in which the signal is to be transmitted contain a relativelyhigh amount of noise. To avoid such noisy channels it is desirable thatthe channel frequency at which a signal from a remote transmitter isbeing transmitted is varied from transmission to transmission.Additionally, it is desirable to vary the channel frequency at which thetransmissions are made from transmission to transmission to minimise thedanger of a signal which is transmitted by the remote transmitter beingpicked up and retransmitted by an unauthorised person, for example, acar thief. The channel frequency at which the transmissions are made maybe sequentially or randomly varied, but more typically, are randomlyvaried.

[0005] In cases where the signal being transmitted is a frequencymodulated signal, in general, there is little difficulty in providing aradio receiver for locating in the vehicle which is readily capable ofscanning the predetermined frequency bandwidth containing the channelsin which the transmissions may be made, and then locking onto afrequency of the channel at which a signal has been detected. However,in many cases it is desirable that the signal transmitted by a remotetransmitter should be an AM radio signal. Unfortunately, due to the lackof suitable scanning methods and apparatus for automatically scanning afrequency bandwidth for an AM signal, it has been necessary to set thetransmitter and receiver of vehicle security apparatus to transmit andreceive on one single fixed AM frequency. This is undesirable.

[0006] There is therefore a need for a method and apparatus forreceiving an AM radio signal which may be transmitted in any one of aplurality of frequency channels within a predetermined frequency band.

[0007] The present invention is directed towards providing such a methodand apparatus.

[0008] According to the invention there is provided a method forreceiving an AM radio signal which may be transmitted in any one of aplurality of frequency channels within a predetermined frequencybandwidth, the method comprising operating a tuning means of a radioreceiver to scan the channels of the predetermined frequency bandwidthfor detecting a signal, wherein on detecting a signal in a frequencychannel the tuning means is operated to scan a frequency bandwidthadjacent the channel in which the signal is detected for determining thenumber of channels in which the signal can be detected, and operatingthe tuning means at the frequency of a middle one of the channels inwhich the signal can be detected.

[0009] Preferably, the tuning means is locked onto the frequency of themiddle one of the channels in which the signal can be detected for aperiod sufficient for allowing at least one complete signal to bereceived. Advantageously, the tuning means is locked onto the frequencyof the middle one of the channels in which the signal can be detectedfor a predetermined time period.

[0010] In one embodiment of the invention the frequency bandwidths ofthe respective frequency channels are similar.

[0011] In another embodiment of the invention each time a signal isdetected in a frequency channel in a scan the received signal isanalysed for determining if the signal is of a valid type. Preferably,each received signal is analysed for determining if the signal is of avalid type format.

[0012] Ideally, the tuning means is operated for scanning the frequencychannels within the predetermined frequency bandwidth sequentially fromone end of the predetermined bandwidth to the other.

[0013] In one embodiment of the invention the tuning means is operatedfor scanning the frequency channels within the predetermined frequencybandwidth from the channel of lowest frequency to the channel of highestfrequency.

[0014] Preferably, the tuning means is operated for scanning thepredetermined frequency bandwidth by sequentially altering the scannedfrequency in frequency steps. Advantageously, the frequency steps are ofsimilar value. Ideally, the frequency bandwidth of each frequency stepis similar to the frequency bandwidth of the respective frequencychannels.

[0015] In one embodiment of the invention on detecting a signal in oneof the frequency channels, the tuning means is operated to sequentiallyscan the channels from the channel in which the signal is first detecteduntil a channel in which the signal can no longer be detected has beenreached for determining the middle one of the channels in which thesignal can be detected. Preferably, a count is recorded of the number offrequency channels through which the tuning means is stepped from thechannel in which the signal is first detected to the channel in whichthe signal can no longer be detected has been reached for determiningthe middle one of the channels in which the signal can be detected.Advantageously, the number of frequency steps through which the tuningmeans is stepped after the channel in which the signal is first detectedis limited to a predetermined number of steps.

[0016] In one embodiment of the invention the predetermined number ofsteps through which the tuning means is stepped after the channel inwhich the signal is first detected does not exceed ten steps.Preferably, the predetermined number of steps through which the tuningmeans is stepped after the channel in which the signal is first detecteddoes not exceed seven steps. Preferably, the predetermined number ofsteps through which the tuning means is stepped after the channel inwhich the signal is first detected does not exceed five steps.

[0017] In one embodiment of the invention when the channel in which thesignal can no longer be detected has been reached or the tuning meanshas been stepped through the predetermined number of steps from thechannel in which the signal is first detected, the frequency at whichthe tuning means is operating is altered by approximately half thenumber of frequency steps through which the tuning means had beenoperated from the channel in which the signal is first detected.

[0018] In another embodiment of the invention the received signal ineach frequency channel is passed through a variable level noisesuppressing means, and the received signal from the noise suppressingmeans from each frequency channel is analysed for the presence of noise,and on noise being present in the signal, the level of suppression atwhich the noise suppressing means is being operated for the frequencychannel to which the tuning means is tuned is incremented by one levelup to a maximum predetermined number of suppression levels.

[0019] In a further embodiment of the invention when the noisesuppressing means is being operated at the maximum level for thefrequency channel to which the tuning means is tuned, and noise is stillpresent in the signal from that channel, a flag means is set for thatchannel, and that channel is skipped on each scan of the frequencybandwidth for a predetermined number of scans.

[0020] In another embodiment of the invention on no noise being presentin the signal from the noise suppressing means on the channel to whichthe tuning means is tuned, the suppression level at which the noisesuppressing means is being operated is decremented by one suppressionlevel for that channel until the suppression level at which the noisesuppressing means is being operated for that channel is reduced to azero level of suppression.

[0021] Preferably, each time the level of suppression of the noisesuppressing means is altered for each channel, the new level ofsuppression to which the noise suppressing means has been altered isrecorded.

[0022] In one embodiment of the invention each time noise is detected ina signal on a channel, that channel is assigned an integer of a firstvalue which is summed to already assigned integers to that channel todetermine a current integer value for that channel, and on the currentinteger value for any channel reaching one of a number of predeterminedfirst current integer values which correspond to levels of suppressionafter an integer of the first value has been summed to the alreadyassigned integers to that channel, the level of suppression at which thesuppressing means is being operated for that channel is altered to thecorresponding level of suppression.

[0023] In another embodiment of the invention each time no noise isdetected in a signal on a channel, that channel is assigned an integerof a second value which is deducted from already assigned integers tothat channel to determine a current integer value for that channel, andon the current integer value of any channel falling to one of a numberof predetermined second current integer values which correspond tolevels of suppression after an integer of the second value has beendeducted from the already assigned integers to that channel, the levelof suppression at which the suppressing means is being operated for thatchannel is altered to the corresponding level of suppression.

[0024] Preferably, the first value of the integer is greater than thesecond value of the integer for introducing a sufficient level ofhysteresis for avoiding excessive cycling of the suppressing meansbetween respective adjacent levels of suppression. Advantageously, thefirst current integer values are less than the second current integervalues corresponding to each level of suppression for introducing asufficient level of hysteresis for avoiding excessive cycling of thesuppressing means between the respective adjacent levels of suppression.

[0025] Additionally the invention provides a receiving apparatus forreceiving an amplitude modulated radio signal which may be transmittedin any one of a plurality of frequency channels within a predeterminedfrequency bandwidth, the apparatus comprising a receiving means forreceiving the AM radio signal, a tuning means for sequentially tuningthe receiving means to the plurality of frequency channels, a detectingmeans for detecting the presence of a signal, and a control means forcontrolling the tuning means for scanning the frequency bandwidth fordetecting a signal in a frequency channel, wherein the control means isresponsive to the detecting means detecting a signal in a frequencychannel for controlling the tuning means to scan a frequency bandwidthadjacent the frequency channel in which the signal is detected fordetermining the number of channels in which the signal can be detected,and for further operating the tuning means at the frequency of a middleone of the channels in which the signal can be detected.

[0026] In one embodiment of the invention the control means locks thetuning means onto the frequency of the middle one of the channels inwhich the signal can be detected for a period sufficient for allowing atleast one complete signal to be received. Preferably, the control meanslocks the tuning means onto the frequency of the middle one of thechannels in which the signal can be detected for a predetermined timeperiod.

[0027] In one embodiment of the invention a first signal analysing meansis provided for analysing each received signal for determining if thesignal is of a valid type. Preferably, the first signal analysing meansanalyses each received signal for determining if the signal is of avalid type format.

[0028] Preferably, the control means operates the tuning means forscanning the frequency channels of the predetermined frequency bandwidthsequentially from one end of the predetermined bandwidth to the other.

[0029] In one embodiment of the invention the control means operates thetuning means for scanning the frequency channels of the predeterminedfrequency bandwidth sequentially from the end of lowest frequency to theend of highest frequency.

[0030] In another embodiment of the invention the control means operatesthe tuning means for scanning the predetermined frequency bandwidth bysequentially altering the scanned frequency in respective frequencysteps. Preferably, the control means operates the tuning means forscanning the predetermined frequency bandwidth by altering the scannedfrequency in frequency steps of predetermined frequency. Advantageously,the control means operates the tuning means for scanning thepredetermined frequency bandwidth in predetermined frequency steps ofsimilar frequency bandwidth. Ideally, each frequency step corresponds tothe frequency bandwidth of each channel.

[0031] In another embodiment of the invention the control means isresponsive to the detecting means detecting a signal in one of thechannels for operating the tuning means to sequentially scan thechannels from the channel in which the signal is first detected until achannel in which the signal can no longer be detected has been reachedfor determining the middle one of the channels in which the signal canbe detected.

[0032] In a further embodiment of the invention the control means storesa count of the number of channels through which the tuning means isstepped from the first channel in which a signal is detected to thechannel in which the signal can no longer be detected has been reached.Preferably, the control means is responsive to the count of the numberof channels in which the signal can be detected exceeding apredetermined number or to the channel in which the signal can no longerbe detected being reached for altering the frequency at which the tuningmeans is operating to a frequency which is at least one frequency steptowards the channel in which the signal is first detected fordetermining the middle one of the channels in which the signal can bedetected.

[0033] In one embodiment of the invention a variable level noisesuppressing means is provided for suppressing noise in the receivedsignal under the control of the control means, the received signal oneach frequency channel being passed through the noise suppressing means,and a noise analysing means being provided for analysing the receivedsignal on each channel for noise after the signal has been passedthrough the noise suppressing means, the control means being responsiveto the noise analysing means detecting the presence of noise in achannel to which the tuning means is tuned for incrementing the level ofsuppression at which the noise suppressing means is being operated byone level up to a maximum predetermined number of suppression levels.

[0034] In one embodiment of the invention the control means records thelevel of suppression at which the noise suppressing means is beingoperated for each channel in a storing means.

[0035] In another embodiment of the invention the control means isresponsive to the noise suppressing means being operated at the maximumnoise suppression level in a frequency channel to which the tuning meansis tuned for setting a flag means for that channel, and the controlmeans is responsive to a flag means being set for operating the tuningmeans for skipping any channel for which a flag means has been set for apredetermined number of scans.

[0036] In a further embodiment of the invention the control means isresponsive to the noise analysing means failing to detect noise in thesignal from a channel to which the tuning means is tuned for reducingthe level of suppression to which the noise suppressing means is set forthat channel by one level to a minimum of zero levels of suppression.

[0037] In one embodiment of the invention a gate means is provided whichis responsive to a valid type signal being detected for gating the validtype signal from the apparatus.

[0038] In one embodiment of the invention the apparatus is adapted foruse in a security apparatus of a motor vehicle, and a received validtype signal is gated to the security apparatus. Preferably, theapparatus is adapted for controlling an immobilising means forimmobilising the vehicle. Advantageously, the apparatus is adapted forcontrolling a means for controlling a central locking system of themotor vehicle. Ideally, the apparatus is adapted for controlling analarm circuit of the motor vehicle.

[0039] Additionally the invention provides security apparatus for amotor vehicle in which the security apparatus comprises the receivingapparatus according to the invention, and a second analysing means foranalysing each valid type signal gated from the receiving apparatus fordetermining if the valid type signal is a valid signal for the securityapparatus.

[0040] In one embodiment of the invention an immobilising means isprovided for immobilising the vehicle, the immobilising means beingresponsive to a valid signal being received.

[0041] In another embodiment of the invention an alarm circuit isprovided, and the alarm circuit is armed or disarmed in response to avalid signal being received.

[0042] In a further embodiment of the invention the security apparatusis responsive to a valid signal being received for operating a centrallocking system of the vehicle.

[0043] The advantages of the invention are many. A particularlyimportant advantage of the invention is that the receiving apparatusaccording to the invention is particularly suitable for receiving AMradio signals which may be transmitted in any one of a plurality offrequency channels, which may be randomly selected or otherwise, withina predetermined frequency bandwidth. In particular, the receivingapparatus according to the invention receives the AM radio signal on thefrequency channel in which the signal is strongest, in other words, thefrequency channel on which the AM signal has been transmitted, or achannel adjacent to the frequency channel on which the AM radio signalhas been transmitted. A further advantage of the invention is that thereceiving apparatus is a relatively simple and inexpensive receivingapparatus.

[0044] A further advantage of the invention is achieved by providing thenoise analysing means and the noise suppressing means, in that noise inthe received signals in each channel can be readily easily suppressed.Another advantage of the invention is that when the receiving apparatuscomprises the noise suppressing means operated with the first integervalue weighted against the second integer value, and the second currentinteger value weighted against the first current integer value,excessive cycling of the suppressing means between adjacent levels ofsuppression is minimised.

[0045] The invention will be more clearly understood from the followingdescription of a preferred embodiment thereof which is given by way ofexample only with reference to the accompanying drawings in which:

[0046]FIG. 1 is a block representation of receiving apparatus accordingto the invention for receiving an AM radio signal which may betransmitted in any one of a plurality of frequency channels within apredetermined frequency bandwidth,

[0047]FIG. 2 is a block representation of security apparatus forsecuring a motor vehicle which incorporates the receiving apparatus ofFIG. 1,

[0048]FIG. 3 is a flowchart of a software subroutine which controls theapparatus of FIG. 1,

[0049]FIG. 4 is a flowchart of another software subroutine which alsocontrols the apparatus of FIG. 1, and

[0050] FIGS. 5(a) to (e) are typical waveforms of AM radio signals asreceived by the receiving apparatus of FIG. 1.

[0051] Referring to the drawings and in particular to FIG. 1 there isillustrated receiving apparatus according to the invention indicatedgenerally by the reference numeral 1 for receiving an AM radio signalwhich may be transmitted in any one of a plurality of frequencychannels, either randomly selected or otherwise, within a predeterminedfrequency bandwidth. The apparatus 1 is particularly suitable for usewith security apparatus also according to the invention and indicatedgenerally by the reference numeral 2 in FIG. 2 for a motor vehicle. Theapparatus of FIG. 2 will be described in detail below. The AM radiosignal to which the apparatus 1 is adapted to receive is typicallytransmitted by a keyfob transmitter (not shown) which is of relativelysmall size, and is typically suitable for attaching to a key ring, orlocated in the head of a motor vehicle ignition key. The transmitter(not shown) is of the type which has a facility for transmitting anencoded signal on any one of the plurality of frequency channels, and inthis embodiment of the invention thirty-two frequency channels withinthe predetermined frequency bandwidth of 0.55 MHz between 433.65 MHz and434.20 MHz, each frequency channel being of approximately 0.017 MHzbandwidth. Typically, the signal is an encoded signal and comprises apreamble followed immediately by a specific code. The preamble is suchas to be common to many security systems of motor vehicles for allowinga rapid determination as to whether the received signal is a valid typesignal, in other works, is of a valid format. The specific code isunique to one and only one receiving and security apparatus of one motorvehicle. Such signals will be well known to those skilled in the art.Typically, on each transmission a plurality of the signals aretransmitted sequentially in relatively quick succession, and commonly,the number of complete signals transmitted in each transmission may befrom one to five. This aspect of security apparatus for motor vehiclesand signals and the transmission of such signals will be known to thoseskilled in the art.

[0052] Being an AM radio signal the transmitted signal when received maystraddle a number of frequency channels, and typically, may straddle upto five frequency channels, as, for example, can be seen from FIG. 5.Accordingly, the receiving apparatus 1 is operated for scanning theentire frequency bandwidth sequentially in incremental steps offrequency corresponding to the width of each frequency channel. Theapparatus 1 scans the channels sequentially from the lowest frequency,namely, 433.65 MHz to the highest frequency 434.20 MHz, and then returnsto the lowest frequency. Scanning is continued until a valid typesignal, in other words, a signal of valid type format is received on oneof the frequency channels. Once a signal of valid type format isreceived on one of the frequency channels sequential scanning continuesof the frequency channels until the first of the channels in which thevalid type signal can no longer be detected is reached. The receivingapparatus 1 is then operated to receive the valid type signal at afrequency of a middle one of the channels in which the valid type signalhas been detected, or at a frequency of a channel adjacent the middleone of the channels in which the valid type signal has been detected, aswill be described in detail below. In this way the signal is received inthe channel in which the signal is strongest.

[0053] Turning now to the receiving apparatus 1, the receiving apparatus1 comprises a tuner, namely, a tuner 3 which receives radio signals froman aerial 5. The tuner 3 is operated under the control of a controlmeans, namely, a microprocessor 6 for stepping the tuner 3 through thepredetermined frequency bandwidth of 0.55 MHz from the lowest frequencyof 433.65 MHz to the highest frequency 434.20 MHz through thirty-twoincremental steps each of approximately 0.017 MHz for scanning thethirty-two channels on which the AM radio signal may be transmitted.

[0054] A noise analysing means, namely, a noise analysing circuit 8 isprovided to ascertain if the channel to which the tuner 3 is tunedcontains noise. The output of the noise analysing circuit 8 is read bythe microprocessor 6, which if noise is detected operates a variablelevel noise suppressing means, namely, a variable level noisesuppression circuit 9. The noise suppression circuit 9 is operable atfour levels of noise suppression for correspondingly reducing thesensitivity at which signals are received on the respective channels bythe apparatus 1, for in turn reducing the level of noise in the receivedsignal received on the respective channels. The microprocessor 6operates the noise suppression circuit 9 at the respective noisesuppression levels for reducing the sensitivity of the apparatus 1 indecremental steps for each channel until noise has been substantiallyeliminated from the signal received in those channels in which noise isdetected, and the received signal can be deciphered. In the event thatthe noise suppression circuit 9 is operating at its maximum level ofnoise suppression in any of the channels, and the noise is still presentin that channel, the microprocessor sets a flag means, namely, a jumpflag for that channel which causes the tuner 3 to skip that channelduring subsequent scans of the frequency band as will be describedbelow.

[0055] The microprocessor 6 under the control of suitable software readsthe output of the noise suppression circuit 9, and acts as a detectingmeans for detecting the presence of a signal on the channel to which thetuner 3 is tuned.

[0056] A first signal analysing means comprising a first validationcircuit 10 reads and analyses the signal from the noise suppressioncircuit 9 to ascertain if the signal is of a valid type. The firstvalidation circuit 10 compares the preamble of the received signal witha stored reference preamble to ascertain if the signal is of valid typeand of valid format. On detecting the presence of a signal themicroprocessor 6 reads the output from the first validation circuit 10,and on a valid type signal being received the microprocessor 6 recordsthe channel in which the valid type signal has been received. Themicroprocessor 6 then operates the tuner 3 to continue to step throughthe channels until a channel is reached in which the valid type signalcan no longer be detected, and a count is made of the number of channelsthrough which the tuner 3 is stepped. The microprocessor 6 as will bedescribed below determines from the count of the channels the middlechannel of the channels in which the valid signal has been detected, andlocks the tuner onto the frequency of the middle channel.Simultaneously, the microprocessor 6 operates a gate circuit 11 foroutputting the signal to the security apparatus 2 for further analysis.The microprocessor 6 locks the tuner 6 onto the middle channel frequencyand holds the gate circuit 11 open for a predetermined time periodsufficient to output at least one complete signal to the securityapparatus 2, which in this embodiment of the invention is approximately100 milliseconds. Each time the tuner 3 is incremented to the nextchannel the signal is analysed by the noise analysing circuit 8 and thefirst validation circuit 10.

[0057] Turning now to FIG. 2 the security apparatus 2 incorporates thereceiving apparatus 1 and shares the microprocessor 6 with the receivingapparatus 1 the security apparatus 2 comprises a second signal analysingmeans, namely, a second validation circuit 14 for validating the signalreceived through the gate circuit 11 as being a valid signal for thesecurity apparatus 2 or otherwise. The second validation circuit 14compares one complete signal received through the gate circuit 11 with astored reference signal to again determine if the signal is of validtype format, and then if the code following the preamble is identical toa reference code of the stored reference signal. The microprocessor 6reads the output from the second validation circuit 14, and on thesignal being determined as being a valid signal for the securityapparatus 2 the microprocessor 6 operates drivers 15, 16 and 17 forchanging the state of a vehicle immobiliser in the vehicle, the centrallocking system of the motor vehicle and an alarm circuit of the motorvehicle, respectively, none of which are illustrated, but will be wellknown to those skilled in the art. Thus, if the vehicle immobiliser, thecentral locking system and the alarm circuit are in the armed state, inother words, with the vehicle immobilised, locked and the alarm circuitarmed, reception of a valid signal causes the microprocessor 6 tooperate the drivers 15, 16 and 17 for disarming the vehicle immobiliser,the central locking system and the alarm circuit, and vice versa.

[0058] Referring now to FIG. 3 a flowchart of a subroutine of softwarefor controlling the microprocessor 6 for operating the tuner 3 after avalid type signal has been detected in one of the channels during a scanwill now be described. For convenience the channel in which a valid typesignal has first been detected will be referred to as Channel No. 0,although, it will be appreciated that Channel No. 0 may be any one ofthe thirty-two channels within the predetermined frequency bandwidth.The microprocessor 6 under the control of suitable software continuouslyor at appropriate periodic intervals operates the tuner 3 tosequentially scan the channels in frequency steps of 0.017 MHz whichcorrespond to the bandwidth of each channel, from the lowest frequencyof 433.65 MHz to the highest frequency of 434.20 MHz of thepredetermined frequency bandwidth. The microprocessor 6 operates thetuner 3 to hold the receiving frequency of the receiving apparatus 1 ateach frequency step for a predetermined time interval sufficient toallow the first validation circuit 10 to determine if a received signalis a valid type signal. In this embodiment of the invention thepredetermined time interval is approximately 1 millisecond. On the firstvalidation circuit 10 determining that a valid type signal has beenreceived in a channel, the frequency of which the tuner 3 is beingoperated, the microprocessor 6 calls up the subroutine of FIG. 3.

[0059] Block 20 of the subroutine commences the subroutine which thenmoves to block 21. Block 21 sets a constant N equal to 1 for countingthe channels scanned after Channel No. 0. Block 22 increments thefrequency of the tuner 3 by one frequency step of 0.017 MHz, in thiscase by one frequency step from Channel No. 0 to the next channel up,and for this pass of the subroutine will be referred to as ChannelNo. 1. The subroutine then goes to block 23 which checks if a jump flaghas been set for this channel, and if a jump flag has been set thesubroutine is returned to block 22, and thus skips the channel for whichthe jump flag has been set. Otherwise, the subroutine is moved to block24. Block 24 checks if N is greater than 4. It is expected that thereceived AM signal will not straddle more than five adjacent channels,and thus in this embodiment of the invention the subroutine of FIG. 3 isset to scan to a maximum of four channels after Channel No. 0. In theevent that Block 24 determines that N is greater than 4, the subroutineis moved to block 25 which will be described in detail below. If block24 determines that N is not greater than 4, in other words, that four orless channels from Channel No. 0 have been scanned, the subroutine movesto block 26 which calls up the subroutine of FIG. 4 for operating thenoise suppression circuit 9 as will be described below. The subroutinethen moves to block 27, which operates the first validation circuit 10to analyse the signal to ascertain if the signal is a valid type signal.

[0060] The first validation circuit 10 sets its output high in the eventthat the signal is a valid type signal, and low in the event of thesignal not being a valid type signal. The subroutine then moves to block28 to determine if the signal is a valid type signal. If the output ofthe first validation circuit 10 is high indicating a valid type signal,the subroutine moves to block 29. Block 29 sets the constant N equal toN+1 indicating that the next channel, namely, Channel No. N+1 is next tobe scanned, and returns the subroutine to block 22. Block 22 againincrements the frequency at which the tuner 3 was last operating by afurther frequency step of 0.017 MHz, thus causing the tuner 3 to tune tothe next channel up, namely, Channel No. N+1, and the subroutinecontinues.

[0061] Returning now to block 28, should block 28 detect a low on theoutput of the first validation circuit 10 indicating that a valid typesignal has not been detected in the channel to which the tuner 3 istuned, the subroutine is moved to block 30. Block 30 checks if the valueof N is less than 2, which would indicate that only one channel next toChannel No. 0 had been scanned before the signal disappeared, in otherwords, the signal would have disappeared in Channel No. 1. Thus, thesignal would be determined as existing in one channel only, namely,Channel No. 0, see FIG. 5(a), which illustrates a plot of the amplitudeof the received signal against frequency channels. If block 30determines that N is less than 2 the subroutine moves to block 31 whichdecrements the frequency at which the tuner 3 in currently operating byone frequency step of 0.017 MHz. This, thus, returns the tuner tooperate at Channel No. 0, namely, the only channel in which the validtype signal has been detected, assuming a channel had not been skippedby block 23.

[0062] The subroutine then moves to block 32 which checks if a jump flaghas been set in the channel, the frequency of which has been determinedby block 31. If block 32 determines that a jump flag has not been set,the subroutine moves to block 33 which locks the tuner onto the channel,the frequency of which has been determined by block 31 for thepredetermined time period of 100 milliseconds so that at least onecomplete signal is received and gated through the gate circuit 11, andanalysed by the second validation circuit 14. The subroutine then movesto block 34 which returns control of the microprocessor 36 to thesoftware which is provided for scanning the predetermined frequencybandwidth, which continues sequential scanning of the predeterminedfrequency bandwidth through the thirty-two channels, until a signal isagain detected. If block 32 determines that a jump flag has been set inthe channel, the frequency of which is determined by block 31, thesubroutine moves to block 35 which decrements the frequency by onefrequency step, and returns the subroutine to block 32. The combinationof blocks 32 and 35 are required due to the fact that block 23 of thesubroutine skips channels in which a jump flag has been set, and thusthe combination of blocks 32 and 35 compensate for block 23.

[0063] Should block 30 determine that N is not less than 2, in otherwords, is equal to 2 or greater, the subroutine is moved to block 36.Block 36 checks if N is less than 4 which would indicate that one or twoadditional channels had been scanned beyond Channel No. 0 before thesignal disappeared. This, thus, would indicate that the signal straddledtwo or three channels, namely, Channels Nos. 0 and 1 or Channels Nos. 0,1 and 2, see FIGS. 5(b) and 5(c), which illustrate a plot of theamplitude of the received signal against frequency channels. If block 36determines that N is less than 4 the subroutine is moved to block 37which decrements the frequency at which the tuner 3 is currentlyoperating by two frequency steps, namely, by 0.034 MHz. This, thus,causes the tuner 3 to operate at two channels below the channel at whichthe signal disappeared. Thereby, in the case of the signal straddlingthree channels, namely, Channels Nos. 0, 1 and 2, the tuner 3 isoperated at the frequency of the middle channel, namely, Channel No. 1,see FIG. 5(c), assuming no channels have been skipped by block 23. Onthe other hand, if the signal straddles two channels, namely, ChannelsNos. 0 and 1 the tuner 3 is operated at the frequency of Channel No. 0,which is one of the channels adjacent the middle of the two ChannelsNos. 0 and 1, see FIG. 5(b), assuming no channels have been skipped byblock 23. The subroutine then moves to block 32, which has already beendescribed, and which if a channel had been skipped by block 23, and ifthe skipping of the channel resulted in the channel selected by block 37being a channel for which a jump flag is set, in combination with block35 selects the first channel below the channel selected by block 37 forwhich a jump flag has not been set. Otherwise, block 32 moves thesubroutine to block 33, and then to block 34.

[0064] In the event that block 36 determines that N is not less than 4,this indicates that two additional channels beyond Channel No. 2 havebeen scanned, namely Channels Nos. 3 and 4. This, thus, indicates thatthe signal straddles four or five channels, namely, Channels Nos. 0 to3, or Channels Nos. 0 to 4, see FIGS. 5(d) and 5(e), which illustrate aplot of amplitude of the received signal against frequency channels. Ifblock 36 determines that N is not less than 4, the subroutine is movedto block 25. Block 25 decrements the frequency at which the tuner 3 isoperating by three frequency steps, namely, 0.051 MHz which causes thetuner 3 to operate in a channel which is three channels below thechannel in which the signal disappeared, assuming no jump flags havebeen skipped by block 23. Thus, in the case of the signal straddlingfive channels, namely, Channels Nos. 0 to 4 the tuner is operated in themiddle channel, namely, Channel No. 2, see FIG. 5. On the other hand, ifthe signal straddles four channels, namely, Channels Nos. 0 to 3, thetuner is operated in Channel No. 1, which is the channel adjacent to themiddle of the four channels, see FIG. 5(d). The subroutine then movesonto block 32, which has already been described, and eventually throughblocks 33 and 34, and depending on block 32 may also pass through block35, and thus, block 33 locks the tuner onto the frequency channeldetermined by either block 25 or 35.

[0065] After block 32 has locked the tuner 3 onto the channel, thefrequency of which is determined by blocks 25, 31, 35 or 37, as the casemay be for the predetermined time period, until one complete signal hasbeen gated through the gate circuit 11, and analysed by the secondvalidation circuit 14, and validated or otherwise, the subroutine thenmoves to block 34 which as already described returns control of themicroprocessor 6 to the software which is provided for scanning thepredetermined frequency bandwidth, which continues sequential scanningof the predetermined frequency bandwidth through the thirty-two channelsuntil a signal is again detected.

[0066] Turning now to FIG. 4 there is illustrated a flow chart of asubroutine of software under which the microprocessor 6 is operated forcontrolling the noise suppression circuit 9 for reducing the sensitivityat which the receiving apparatus 1 receives signals in the respectivechannels of the frequency bandwidth. This subroutine of FIG. 4 is calledup by block 26 of the subroutine of FIG. 3 as each channel is beingscanned. A storing means provided by a noise level register in themicroprocessor 6 stores the current suppression level at which the noisesuppression circuit 9 is being operated for each channel, and an integerregister in the microprocessor 6 also stores the value of an integer Ifor each channel which will be described below.

[0067] The integer I is used for determining when the noise suppressionlevel should be varied up or down for each channel. The integer Iprovides a pseudo-count of the number of scans of each channel in whichnoise is or is not detected by the analysing circuit 8 after the noisesuppression level for that channel has been incremented or decremented.The number of counts in which noise is detected is weighted over andabove the number of counts in which noise is not detected. On each scanin which noise is detected in a signal on a channel after the level ofnoise suppression for that channel has been altered, the integer I forthat channel is incremented by a first value, which in this embodimentof the invention is 5, each time no noise is detected, the integer I isdecremented by a second value, which in this embodiment of the inventionis 1. When the value of the integer I is increasing and it reaches afirst one of four first current integer values, which correspond to thefour levels of suppression, respectively, the noise suppression circuit9 is set to the first level of noise suppression, namely, suppressionlevel 1. The first of the four first current values of the integer I inthis embodiment of the invention is 20. On the value of the integer Ireaching the second of the four first current integer values, which inthis embodiment of the invention is 40, the noise suppression circuit 9is set to the second level of noise suppression, namely, noisesuppression level 2. On the value of the integer I reaching the third ofthe four first current integer values, which in this embodiment of theinvention is 60, the noise suppression circuit 9 is set to the thirdlevel of noise suppression, namely, noise suppression level 3. On theinteger I reaching the fourth of the first current integer values, whichin this embodiment of the invention is 80, the noise suppression circuit9 is set to the fourth level of noise suppression, namely, noisesuppression level 4. Should noise remain in the signal after the noisesuppression level has been set to the level 4 and the signal cannot bedeciphered, the jump flag is set as will be described below for causingthe microprocessor 6 to cause the tuner to jump that channel during eachscan of the frequency bandwidth for a predetermined number of scans,which in this embodiment of the invention is 5 scans.

[0068] When the value of the integer I is reducing, and it falls to afirst one of four second current integer values, which also correspondto the four levels of noise suppression, respectively, the noisesuppression circuit 9 is set to the third noise suppression level,namely, level 3. In this embodiment of the invention the first one ofthe values of the second current integer values is 75. On the value ofthe integer I falling to the second value of the second current integervalues, which in this embodiment of the invention is 55, the noisesuppression circuit 9 is set to level 2. On the value of the integer Ifalling to the third of the second current integer values, which in thisembodiment of the invention is 35, the noise suppression circuit 9 isset to level 1. On the value of the integer I falling to the fourth ofthe second current integer values, which in this embodiment of theinvention is 15, the noise suppression circuit 9 is operated to removenoise suppression from that channel.

[0069] Referring now in particular to FIG. 4, block 40 starts thesubroutine and the subroutine then moves to block 41. Block 41 reads thenoise suppression level from the register in the microprocessor 6 atwhich the noise suppression circuit 9 is currently set for the channelto which the tuner 3 is tuned. The subroutine then moves to block 42which reads the current value of the integer I from the integer registerfor that channel. The subroutine then moves to block 51 which reads theoutput of the noise analysing circuit 8, and determines if noise ispresent in the signal. If noise is present in the signal the subroutinemoves to block 52 which increments the value of the integer I by 5. Thesubroutine then moves to block 43 which stores the new value of theinteger I in the integer register, and moves to block 53. Block 53checks if the value of I is greater than or equal to 85. If the value ofthe integer I is not greater than or equal to 85 the subroutine moves toblock 54 which checks if the value of I is greater than or equal to 80.If so, the subroutine moves to block 55 which sets the suppression levelof the noise suppression circuit 9 at level No. 4. The subroutine thenmoves to block 44 which stores the new noise suppression level in theappropriate register for that channel. The subroutine then moves toblock 56 which exits from the subroutine and returns control of themicroprocessor 6 to the subroutine of FIG. 3. On the other hand shouldblock 54 determine that I is less than 80 the subroutine moves to block57, which checks if the value of I is greater than or equal to 60. Ifso, the subroutine moves to block 58 which sets the level of suppressionof the noise suppression circuit 9 at level No. 3, and the subroutinethen moves to block 44 which has already been described. On the otherhand should block 57 determine that I is less than 60, the subroutinemoves to block 59 which checks if the value of I is greater than orequal to 40. If so, the subroutine moves to block 60 which sets thesuppression level of the noise suppression circuit 9 to level No. 2. Thesubroutine then moves to block 44 which has already been described.Should block 59 determine that the value of I is less than 40 thesubroutine then moves to block 61 which checks if the value of I isgreater than or equal to 20. If so, the subroutine moves to block 62which sets the suppression level of the noise suppression circuit 9 atlevel No. 1 and the subroutine moves to block 44. Should block 61determine that I is less than 20 the subroutine moves to block 56.

[0070] Should block 53 determine that the value of the integer I isgreater than or equal to 85 thus indicating that noise is still presentin the signal after the noise suppression circuit 9 has been set at themaximum noise suppression level 4 on the last frequency bandwidth scanthe subroutine moves to block 65. Block 65 sets the jump flag for thatchannel, thereby causing the tuner to skip that channel for thepredetermined number of 5 scans of the frequency bandwidth. Thesubroutine then moves to block 44 which has already been described.

[0071] On the other hand, should block 51 determine that no noise can bedetected in the signal the subroutine moves to block 66 which decrementsthe value of the integer I by 1. The subroutine then moves to block 45which stores the new value of the integer I for that channel in theinteger register, and the subroutine moves to block 67. Block 67 checksif the value of the integer I is less than or equal to 15, and if so,the subroutine moves to block 68 which causes the noise suppressioncircuit 9 to operate with no noise suppression on the signal. In otherwords, the apparatus 1 is operated at the maximum level of sensitivityfor receiving the signals on that particular channel. The subroutinethen moves to block 46 which stores the new value of the noisesuppression level for that channel in the noise level register, and thesubroutine moves to block 56 which has already been described. Shouldblock 67 determine that the value of the integer I is not less than orequal to 15 the subroutine moves to block 69 which checks if the valueof the integer I is less than or equal to 35. If so, the subroutinemoves to block 70 which decreases the level of noise suppression,thereby causing the noise suppression circuit to operate at level 1, andthe subroutine moves to block 46. Should block 69 determine that thevalue of the integer I is not less than or equal to 35, the subroutinecontinues to pass through blocks 71 and 72 in similar fashion to thatthrough which the subroutine passed through the blocks 57 and 59. Ifblock 71 determines that I is less than or equal to 55 the subroutinemoves to block 74 which sets the noise level at level No. 3. If block 72determines that I is less than or equal to 75 the subroutine moves toblock 75 which sets the suppression level at level No. 3. Should thesubroutine reach block 73, and block 73 determines that the value of theinteger I is less than or equal to 84 the subroutine moves to block 76which removes the jump flag, and moves the subroutine to block 46. Onthe other hand, should block 73 determine that the value of the integerI is not less than or equal to 84 the subroutine moves to block 77 whichresets the jump flag so that this channel will continue to be jumped fora further predetermined number of scans of the frequency bandwidth,namely, 5 scans. The subroutine then moves to block 56.

[0072] By incrementing the value of the integer I by the firstpredetermined integer value of 5 on each scan of the channel in whichnoise is detected and increasing the level of suppression by one levelat each of the values of the first current integer values of the integerI being equal to 20, 40, 60 and 80, and by decrementing the value of Iby the second value of 1, which is less than the first integer value oneach scan of the channel in which no noise is detected, and by reducingthe noise suppression level by one level each time the value of thesecond current integer value of the integer I is reduced to 15, 35, 55and 75 introduces a sufficient level of hysteresis into the noisesuppression circuit of the apparatus 1 to avoid excessive cycling of thenoise suppression circuit between adjacent levels of noise suppression.

[0073] While the method for determining the level of noise suppressionto be applied to each channel has been described as being determined bythe value of the integer I, other suitable means for determining thelevel of suppression to be applied and for determining the appropriatetimes at which to apply the level of suppression may be used. It is alsoenvisaged that where the level of suppression to be applied to eachchannel is determined by the value of the integer I, other first andsecond values of the integer I, and other first and second currentinteger values could be used besides those described.

[0074] It will also be appreciated that other methods for determiningwhen the jump flag is to be set may be used, and it will also beappreciated that the predetermined number of scans during which achannel, a jump flag for which has been set, may be other than five.

[0075] In certain cases, it is envisaged that a jump flag would not beset, and none of the channels would be skipped during scanning of thepredetermined frequency bandwidth.

1. A method for receiving an AM radio signal which may be transmitted inany one of a plurality of frequency channels within a predeterminedfrequency bandwidth, the method comprising operating a tuning means (3)of a radio receiver to scan the channels of the predetermined frequencybandwidth for detecting a signal, characterised in that on detecting asignal in a frequency channel the tuning means (3) is operated to scan afrequency bandwidth adjacent the channel in which the signal is detectedfor determining the number of channels in which the signal can bedetected, and operating the tuning means (3) at the frequency of amiddle one of the channels in which the signal can be detected.
 2. Amethod as claimed in claim 1 characterised in that the tuning means (3)is locked onto the frequency of the middle one of the channels in whichthe signal can be detected for a period sufficient for allowing at leastone complete signal to be received.
 3. A method as claimed in claim 2characterised in that the tuning means (3) is locked onto the frequencyof the middle one of the channels in which the signal can be detectedfor a predetermined time period.
 4. A method as claimed in any precedingclaim characterised in that the frequency bandwidths of the respectivefrequency channels are similar.
 5. A method as claimed in any precedingclaim characterised in that each time a signal is detected in afrequency channel in a scan the received signal is analysed fordetermining if the signal is of a valid type.
 6. A method as claimed inclaim 5 characterised in that each received signal is analysed fordetermining if the signal is of a valid type format.
 7. A method asclaimed in any preceding claim characterised in that the tuning means(3) is operated for scanning the frequency channels within thepredetermined frequency bandwidth sequentially from one end of thepredetermined bandwidth to the other.
 8. A method as claimed in claim 6characterised in that the tuning means (3) is operated for scanning thefrequency channels within the predetermined frequency bandwidth from thechannel of lowest frequency to the channel of highest frequency.
 9. Amethod as claimed in claim 7 or 8 characterised in that the tuning means(3) is operated for scanning the predetermined frequency bandwidth bysequentially altering the scanned frequency in frequency steps.
 10. Amethod as claimed in claim 9 characterised in that the frequency stepsare of similar value.
 11. A method as claimed in claim 9 or 10characterised in that the frequency bandwidth of each frequency step issimilar to the frequency bandwidth of the respective frequency channels.12. A method as claimed in any preceding claim characterised in that ondetecting a signal in one of the frequency channels, the tuning means(3) is operated to sequentially scan the channels from the channel inwhich the signal is first detected until a channel in which the signalcan no longer be detected has been reached for determining the middleone of the channels in which the signal can be detected.
 13. A method asclaimed in claim 12 characterised in that a count is recorded of thenumber of frequency channels through which the tuning means (3) isstepped from the channel in which the signal is first detected to thechannel in which the signal can no longer be detected has been reachedfor determining the middle one of the channels in which the signal canbe detected.
 14. A method as claimed in claim 13 characterised in thatthe number of frequency steps through which the tuning means (3) isstepped after the channel in which the signal is first detected islimited to a predetermined number of steps.
 15. A method as claimed inclaim 14 characterised in that the predetermined number of steps throughwhich the tuning means (3) is stepped after the channel in which thesignal is first detected does not exceed ten steps.
 16. A method asclaimed in claim 15 characterised in that the predetermined number ofsteps through which the tuning means (3) is stepped after the channel inwhich the signal is first detected does not exceed seven steps.
 17. Amethod as claimed in claim 16 characterised in that the predeterminednumber of steps through which the tuning means (3) is stepped after thechannel in which the signal is first detected does not exceed fivesteps.
 18. A method as claimed in any of claims 12 to 16 characterisedin that when the channel in which the signal can no longer be detectedhas been reached or the tuning means (3) has been stepped through thepredetermined number of steps from the channel in which the signal isfirst detected, the frequency at which the tuning means (3) is operatingis altered by approximately half the number of frequency steps throughwhich the tuning means (3) had been operated from the channel in whichthe signal is first detected.
 19. A method as claimed in any precedingclaim characterised in that the received signal in each frequencychannel is passed through a variable level noise suppressing means (9),and the received signal from the noise suppressing means (9) from eachfrequency channel is analysed for the presence of noise, and on noisebeing present in the signal, the level of suppression at which the noisesuppressing means (9) is being operated for the frequency channel towhich the tuning means (3) is tuned is incremented by one level up to amaximum predetermined number of suppression levels.
 20. A method asclaimed in claim 19 characterised in that when the noise suppressingmeans (9) is being operated at the maximum level for the frequencychannel to which the tuning means (3) is tuned, and noise is stillpresent in the signal from that channel, a flag means is set for thatchannel, and that channel is skipped on each scan of the frequencybandwidth for a predetermined number of scans.
 21. A method as claimedin claim 19 or 20 characterised in that on no noise being present in thesignal from the noise suppressing means (9) on the channel to which thetuning means (3) is tuned, the suppression level at which the noisesuppressing means (9) is being operated is decremented by onesuppression level for that channel until the suppression level at whichthe noise suppressing means (9) is being operated for that channel isreduced to a zero level of suppression.
 22. A method as claimed in anyof claims 19 to 21 characterised in that each time the level ofsuppression of the noise suppressing means (9) is altered for eachchannel, the new level of suppression to which the noise suppressingmeans (9) has been altered is recorded.
 23. A method as claimed in anyof claims 19 to 22 characterised in that each time noise is detected ina signal on a channel, that channel is assigned an integer of a firstvalue which is summed to already assigned integers to that channel todetermine a current integer value for that channel, and on the currentinteger value for any channel reaching one of a number of predeterminedfirst current integer values which correspond to levels of suppressionafter an integer of the first value has been summed to the alreadyassigned integers to that channel, the level of suppression at which thesuppressing means (9) is being operated for that channel is altered tothe corresponding level of suppression.
 24. A method as claimed in anyof claims 19 to 23 characterised in that each time no noise is detectedin a signal on a channel, that channel is assigned an integer of asecond value which is deducted from already assigned integers to thatchannel to determine a current integer value for that channel, and onthe current integer value of any channel falling to one of a number ofpredetermined second current integer values which correspond to levelsof suppression after an integer of the second value has been deductedfrom the already assigned integers to that channel, the level ofsuppression at which the suppressing means (9) is being operated forthat channel is altered to the corresponding level of suppression.
 25. Amethod as claimed in claim 23 or 24 characterised in that the firstvalue of the integer is greater than the second value of the integer forintroducing a sufficient level of hysteresis for avoiding excessivecycling of the suppressing means (9) between respective adjacent levelsof suppression.
 26. A method as claimed in any of claims 23 to 25characterised in that the first current integer values are less than thesecond current integer values corresponding to each level of suppressionfor introducing a sufficient level of hysteresis for avoiding excessivecycling of the suppressing means (9) between the respective adjacentlevels of suppression.
 27. A receiving apparatus for receiving anamplitude modulated radio signal which may be transmitted in any one ofa plurality of frequency channels within a predetermined frequencybandwidth, the apparatus (1) comprising a receiving means (1) forreceiving the AM. radio signal, a tuning means (3) for sequentiallytuning the receiving means (1) to the plurality of frequency channels, adetecting means (6) for detecting the presence of a signal, and acontrol means (6) for controlling the tuning means (3) for scanning thefrequency bandwidth for detecting a signal in a frequency channel,characterised in that the control means (6) is responsive to thedetecting means (6) detecting a signal in a frequency channel forcontrolling the tuning means (3) to scan a frequency bandwidth adjacentthe frequency channel in which the signal is detected for determiningthe number of channels in which the signal can be detected, and forfurther operating the tuning means (3) at the frequency of a middle oneof the channels in which the signal can be detected.
 28. Apparatus asclaimed in claim 27 characterised in that the control means (6) locksthe tuning means (3) onto the frequency of the middle one of thechannels in which the signal can be detected for a period sufficient forallowing at least one complete signal to be received.
 29. Apparatus asclaimed in claim 28 characterised in that the control means (6) locksthe tuning means (3) onto the frequency of the middle one of thechannels in which the signal can be detected for a predetermined timeperiod.
 30. Apparatus as claimed in any of claims 27 to 29 characterisedin that a first signal analysing means (10) is provided for analysingeach received signal for determining if the signal is of a valid type.31. Apparatus as claimed in claim 30 characterised in that the firstsignal analysing means (10) analyses each received signal fordetermining if the signal is of a valid type format.
 32. Apparatus asclaimed in any of claims 27 to 31 characterised in that the controlmeans (6) operates the tuning means (3) for scanning the frequencychannels of the predetermined frequency bandwidth sequentially from oneend of the predetermined bandwidth to the other.
 33. Apparatus asclaimed in claim 32 characterised in that the control means (6) operatesthe tuning means for scanning the frequency channels of thepredetermined frequency bandwidth sequentially from the end of lowestfrequency to the end of highest frequency.
 34. Apparatus as claimed inany of claims 27 to 33 characterised in that the control means (6)operates the tuning means (3) for scanning the predetermined frequencybandwidth by sequentially altering the scanned frequency in respectivefrequency steps.
 35. Apparatus as claimed in claim 34 characterised inthat the control means (6) operates the tuning means (3) for scanningthe predetermined frequency bandwidth by altering the scanned frequencyin frequency steps of predetermined frequency.
 36. Apparatus as claimedin claim 35 characterised in that the control means (6) operates thetuning means (3) for scanning the predetermined frequency bandwidth inpredetermined frequency steps of similar frequency bandwidth. 37.Apparatus as claimed in any of claims 34 to 36 characterised in thateach frequency step corresponds to the frequency bandwidth of eachchannel.
 38. Apparatus as claimed in any of claims 27 to 37characterised in that the control means (6) is responsive to thedetecting means (6) detecting a signal in one of the channels foroperating the tuning means (3) to sequentially scan the channels fromthe channel in which the signal is first detected until a channel inwhich the signal can no longer be detected has been reached fordetermining the middle one of the channels in which the signal can bedetected.
 39. Apparatus as claimed in any of claims 27 to 38characterised in that the control means (6) stores a count of the numberof channels through which the tuning means (3) is stepped from the firstchannel in which a signal is detected to the channel in which the signalcan no longer be detected has been reached.
 40. Apparatus as claimed inclaim 39 characterised in that the control means (6) is responsive tothe count of the number of channels in which the signal can be detectedexceeding a predetermined number or to the channel in which the signalcan no longer be detected being reached for altering the frequency atwhich the tuning means (3) is operating to a frequency which is at leastone frequency step towards the channel in which the signal is firstdetected for determining the middle one of the channels in which thesignal can be detected.
 41. Apparatus as claimed in any of claims 27 to40 characterised in that a variable level noise suppressing means (9) isprovided for suppressing noise in the received signal under the controlof the control means (6), the received signal on each frequency channelbeing passed through the noise suppressing means (9), and a noiseanalysing means (8) being provided for analysing the received signal oneach channel for noise after the signal has been passed through thenoise suppressing means (9), the control means being responsive to thenoise analysing means (8) detecting the presence of noise in a channelto which the tuning means (3) is tuned for incrementing the level ofsuppression at which the noise suppressing means (9) is being operatedby one level up to a maximum predetermined number of suppression levels.42. Apparatus as claimed in claim 41 characterised in that the controlmeans (6) records the level of suppression at which the noisesuppressing means (9) is being operated for each channel in a storingmeans.
 43. Apparatus as claimed in claim 41 or 42 characterised in thatthe control means (6) is responsive to the noise suppressing means (9)being operated at the maximum noise suppression level in a frequencychannel to which the tuning means is tuned for setting a flag means forthat channel, and the control means (6) is responsive to a flag meansbeing set for operating the tuning means (3) for skipping any channelfor which a flag means has been set for a predetermined number of scans.44. Apparatus as claimed in any of claims 41 to 43 characterised in thatthe control means (6) is responsive to the noise analysing means (8)failing to detect noise in the signal from a channel to which the tuningmeans (3) is tuned for reducing the level of suppression to which thenoise suppressing means (9) is set for that channel by one level to aminimum of zero levels of suppression.
 45. Apparatus as claimed in anyof claims 27 to 44 characterised in that a gate means (11) is providedwhich is responsive to a valid type signal being detected for gating thevalid type signal from the apparatus.
 46. Apparatus as claimed in any ofclaims 27 to 45 characterised in that the apparatus (1) is adapted foruse in a security apparatus (2) of a motor vehicle, and a received validtype signal is gated to the security apparatus.
 47. Apparatus as claimedin claim 46 characterised in that the apparatus (1) is adapted forcontrolling an immobilising means for immobilising the vehicle. 48.Apparatus as claimed in claim 46 or 47 characterised in that theapparatus (1) is adapted for controlling a means for controlling acentral locking system of the motor vehicle.
 49. Apparatus as claimed inany of claims 46 to 48 characterised in that the apparatus (1) isadapted for controlling an alarm circuit of the motor vehicle. 50.Security apparatus for a motor vehicle characterised in that thesecurity apparatus comprises the receiving apparatus (1) as claimed inany of claims 27 to 49, and a second analysing means (10) for analysingeach valid type signal gated from the receiving apparatus (1) fordetermining if the valid type signal is a valid signal for the securityapparatus.
 51. Security apparatus as claimed in claim 50 characterisedin that an immobilising means is provided for immobilising the vehicle,the immobilising means being responsive to a valid signal beingreceived.
 52. Security apparatus as claimed in claim 50 or 51characterised in that an alarm circuit is provided, and the alarmcircuit is armed or disarmed in response to a valid signal beingreceived.
 53. Security apparatus as claimed in any of claims 50 to 52characterised in that the security apparatus (2) is responsive to avalid signal being received for operating a central locking system ofthe vehicle.